I'm using Automatic1111's implementation, I'm not sure if this is applicable to other UI's, but when I have things happening that I don't want I do this: Identify what prompt words are creating the issue (open mouth for you), and then introduce that keyword later in the generation. That way, the poses/features of your characters will start 'normal', and once that is set the attributes you want will layer on when you tell it to. Automatic1111's allows for prompt weights with () for positive and [] for negative, but it also lets you drop keywords, replace them, or introduce them mid-render. For example if you wanted a bloody zombie, part of your prompt would look like this - zombie facing the camera [with a bloody face:50], --this adds 'bloody face' at step 50 zombie facing the camera with a [drenched:bloody:50] face, --this will have it have a 'drenched' face up until 50 (think of it as a base layer for the blood), then at 50 'drenched' will be replaced with 'bloody' [word::step] --this will remove a word at the given step, I'm just not sure if it applies to your problem!

I was like this as a child. Got told to close my mouth, stop catching flies, I'm not a codfish, all the fun stuff. Turns out my jaw didn't fit right and my mouth physically couldn't close, lol. It caused huge strain on my jaw tendons to even try, and the effort was ruining my back teeth. My front teeth had never met each other, ever. They did a major realignment surgery of my jaws, and now I can close my mouth. I try to not comment on stuff like that, because you never know what someone's dealing with.

There could be several reasons for your problem. First of all is it your lips that are always open? If so, it could be either you are a mouth breather or you have a short upper lip. If you are a mouth breather you may have chronic nasal congestion, and you should see an ENT physician. If you have a short upper lip, you have a fairly common problem. I am not aware of any surgical correction for this, but in my practice, I gave my patients who had this a simple exercise. Take a coin- a quarter or a half dollar, clean it well, and hold it between your lips. After a while, you will unconsciously open your mouth and the coin will fall. Keep doing this, and you will strengthen the lip muscle, and you will be able to hold the coin for longer and longer. You will then be able to keep your lips closed for longer and longer. It would be a good idea to have a consultation with a dentist or orthodontist.